Moths possess a highly specialized auditory system that plays a significant role in their survival. Their ability to perceive sound is not a general sense like that of humans but a finely tuned sensor optimized for specific, high-frequency signals. This unique hearing capability is a direct result of evolutionary pressure, shaping their anatomy and behavior. The auditory range of many nocturnal moths is focused primarily on ultrasonic frequencies, far above the pitch humans can detect.
Where Moths Locate Their Ears
Moths do not hear using the complex coiled cochlea and external pinna structure found in mammals. Instead, their auditory organs are simple, pressure-sensitive structures called tympanal organs. These organs are essentially thin, drum-like membranes of cuticle, known as the tympanum, stretched over an air-filled sac. When sound waves strike this membrane, the resulting vibrations are detected by a small cluster of sensory nerve cells, or scolophores, attached to the inner surface of the tympanum.
The location of these “ears” varies widely depending on the moth family. Many members of the Noctuoidea superfamily, which includes common owlet and tiger moths, have a pair of tympanal organs located on the side of the metathorax, the final segment of the thorax near the base of the wings. Other groups, such as the Geometridae (geometer moths), house their paired tympanal organs on the first segment of the abdomen. Some hawkmoths (Sphingidae) have even evolved ears near their mouthparts, utilizing modified structures like the labial palps. The simplest of these organs, such as those in noctuid moths, may contain only two sensory cells, yet they are sensitive to the necessary frequencies.
Tuning In to Ultrasound: The Bat-Moth Arms Race
The primary function of moth hearing is not communication but defense against the threat of echolocating bats. This predator-prey dynamic has resulted in a long-running “acoustic arms race” that has driven the evolution of both groups. Moths that are active at night have evolved tympanal organs specifically tuned to the ultrasonic frequencies between 20 and 60 kilohertz (kHz) that bats use for echolocation.
A bat’s hunt involves a sequence of increasingly rapid ultrasonic pulses, culminating in a “terminal buzz” just before capture. Moths have evolved distinct, intensity-dependent behavioral responses to these signals. When a moth detects a faint, distant echolocation signal, the moth typically executes a simple evasive maneuver, steering its flight path away from the sound source. This response is known as negative phonotaxis, moving away from the sound.
If the sound rapidly increases in intensity and pulse repetition rate, resembling a bat’s final attack sequence, the moth’s response becomes far more drastic. The sudden, loud sound triggers a reflex that causes the moth to stop flying and execute erratic, uncoordinated maneuvers, such as power dives or spirals toward the ground. This chaotic flight pattern makes it difficult for the bat’s sonar to calculate the moth’s trajectory, effectively disrupting the final stage of the attack. Some species, like the Luna moth, have long, twisted tails on their hindwings that can deflect the incoming ultrasonic waves, causing the bat to target a non-essential part of the moth’s body.
Auditory Communication Within Moth Species
While predator avoidance is the original and most widespread function of moth hearing, the presence of ears has also facilitated the development of intraspecies communication. This secondary use of sound is distinct from the primary reliance on chemical pheromones for mating. Certain moth species have evolved to produce their own ultrasonic signals, which they use for courtship or as an active defense.
Some male moths, particularly in the tiger moth subfamily (Arctiinae), produce trains of high-frequency clicks using specialized structures called tymbals. These clicks serve a dual purpose: they can signal the male’s location to a receptive female or function as a warning signal to a bat, often indicating that the moth is chemically distasteful (aposematism). The clicks can also be produced rapidly enough to actively confuse or “jam” the bat’s sonar system, a sophisticated acoustic countermeasure.
Other forms of acoustic communication are much quieter, designed to avoid detection by eavesdropping bats. This is seen in “whispering” moths, such as the Asian corn borer, where males produce low-intensity ultrasonic courtship songs. These quiet signals can only be detected by a female when the male is within a few centimeters. The evolution of sound communication in moths is therefore a fine balance between attracting a mate and avoiding a deadly predator.